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Introduce default_field_values feature

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Initial implementation of `#[feature(default_field_values]`, proposed in https://github.com/rust-lang/rfcs/pull/3681.

Support default fields in enum struct variant

Allow default values in an enum struct variant definition:

```rust
pub enum Bar {
    Foo {
        bar: S = S,
        baz: i32 = 42 + 3,
    }
}
```

Allow using `..` without a base on an enum struct variant

```rust
Bar::Foo { .. }
```

`#[derive(Default)]` doesn't account for these as it is still gating `#[default]` only being allowed on unit variants.

Support `#[derive(Default)]` on enum struct variants with all defaulted fields

```rust
pub enum Bar {
    #[default]
    Foo {
        bar: S = S,
        baz: i32 = 42 + 3,
    }
}
```

Check for missing fields in typeck instead of mir_build.

Expand test with `const` param case (needs `generic_const_exprs` enabled).

Properly instantiate MIR const

The following works:

```rust
struct S<A> {
    a: Vec<A> = Vec::new(),
}
S::<i32> { .. }
```

Add lint for default fields that will always fail const-eval

We *allow* this to happen for API writers that might want to rely on users'
getting a compile error when using the default field, different to the error
that they would get when the field isn't default. We could change this to
*always* error instead of being a lint, if we wanted.

This will *not* catch errors for partially evaluated consts, like when the
expression relies on a const parameter.

Suggestions when encountering `Foo { .. }` without `#[feature(default_field_values)]`:

 - Suggest adding a base expression if there are missing fields.
 - Suggest enabling the feature if all the missing fields have optional values.
 - Suggest removing `..` if there are no missing fields.
This commit is contained in:
Esteban Küber
2024-08-24 17:22:48 +00:00
parent f6cb952dc1
commit 9ac95c10c0
70 changed files with 1469 additions and 392 deletions
Download Patch File Download Diff File Expand all files Collapse all files

71
compiler/rustc_hir/src/hir.rs
View File

@@ -1857,7 +1857,12 @@ impl Expr<'_> {
base.can_have_side_effects()
}
ExprKind::Struct(_, fields, init) => {
fields.iter().map(|field| field.expr).chain(init).any(|e| e.can_have_side_effects())
let init_side_effects = match init {
StructTailExpr::Base(init) => init.can_have_side_effects(),
StructTailExpr::DefaultFields(_) | StructTailExpr::None => false,
};
fields.iter().map(|field| field.expr).any(|e| e.can_have_side_effects())
|| init_side_effects
}
ExprKind::Array(args)
@@ -1926,20 +1931,52 @@ impl Expr<'_> {
ExprKind::Path(QPath::Resolved(None, path2)),
) => path1.res == path2.res,
(
ExprKind::Struct(QPath::LangItem(LangItem::RangeTo, _), [val1], None),
ExprKind::Struct(QPath::LangItem(LangItem::RangeTo, _), [val2], None),
ExprKind::Struct(
QPath::LangItem(LangItem::RangeTo, _),
[val1],
StructTailExpr::None,
),
ExprKind::Struct(
QPath::LangItem(LangItem::RangeTo, _),
[val2],
StructTailExpr::None,
),
)
| (
ExprKind::Struct(QPath::LangItem(LangItem::RangeToInclusive, _), [val1], None),
ExprKind::Struct(QPath::LangItem(LangItem::RangeToInclusive, _), [val2], None),
ExprKind::Struct(
QPath::LangItem(LangItem::RangeToInclusive, _),
[val1],
StructTailExpr::None,
),
ExprKind::Struct(
QPath::LangItem(LangItem::RangeToInclusive, _),
[val2],
StructTailExpr::None,
),
)
| (
ExprKind::Struct(QPath::LangItem(LangItem::RangeFrom, _), [val1], None),
ExprKind::Struct(QPath::LangItem(LangItem::RangeFrom, _), [val2], None),
ExprKind::Struct(
QPath::LangItem(LangItem::RangeFrom, _),
[val1],
StructTailExpr::None,
),
ExprKind::Struct(
QPath::LangItem(LangItem::RangeFrom, _),
[val2],
StructTailExpr::None,
),
) => val1.expr.equivalent_for_indexing(val2.expr),
(
ExprKind::Struct(QPath::LangItem(LangItem::Range, _), [val1, val3], None),
ExprKind::Struct(QPath::LangItem(LangItem::Range, _), [val2, val4], None),
ExprKind::Struct(
QPath::LangItem(LangItem::Range, _),
[val1, val3],
StructTailExpr::None,
),
ExprKind::Struct(
QPath::LangItem(LangItem::Range, _),
[val2, val4],
StructTailExpr::None,
),
) => {
val1.expr.equivalent_for_indexing(val2.expr)
&& val3.expr.equivalent_for_indexing(val4.expr)
@@ -2096,7 +2133,7 @@ pub enum ExprKind<'hir> {
///
/// E.g., `Foo {x: 1, y: 2}`, or `Foo {x: 1, .. base}`,
/// where `base` is the `Option<Expr>`.
Struct(&'hir QPath<'hir>, &'hir [ExprField<'hir>], Option<&'hir Expr<'hir>>),
Struct(&'hir QPath<'hir>, &'hir [ExprField<'hir>], StructTailExpr<'hir>),
/// An array literal constructed from one repeated element.
///
@@ -2111,6 +2148,19 @@ pub enum ExprKind<'hir> {
Err(rustc_span::ErrorGuaranteed),
}
#[derive(Debug, Clone, Copy, HashStable_Generic)]
pub enum StructTailExpr<'hir> {
/// A struct expression where all the fields are explicitly enumerated: `Foo { a, b }`.
None,
/// A struct expression with a "base", an expression of the same type as the outer struct that
/// will be used to populate any fields not explicitly mentioned: `Foo { ..base }`
Base(&'hir Expr<'hir>),
/// A struct expression with a `..` tail but no "base" expression. The values from the struct
/// fields' default values will be used to populate any fields not explicitly mentioned:
/// `Foo { .. }`.
DefaultFields(Span),
}
/// Represents an optionally `Self`-qualified value/type path or associated extension.
///
/// To resolve the path to a `DefId`, call [`qpath_res`].
@@ -3172,6 +3222,7 @@ pub struct FieldDef<'hir> {
pub def_id: LocalDefId,
pub ty: &'hir Ty<'hir>,
pub safety: Safety,
pub default: Option<&'hir AnonConst>,
}
impl FieldDef<'_> {